Ignition Delay Time Measurements Research Articles

The Ignition of C1–C7 Natural Gas Blends and the Effect of Hydrogen Addition in the Low and High Temperature Regimes

Abstract New ignition delay time (IDT) measurements for two natural gas (NG) blends composed of C1–C7n-alkanes, NG6 (C1:60.625%, C2:20%, C3:10%, C4:5%, nC5:2.5%, nC6:1.25%, nC7:0.625%) and NG7 (C1:72.635%, C2:10%, C3:6.667%, C4:4.444%, nC5:2.965%, nC6:1.976%, nC7:1.317%) by volume with methane as the major component are presented. The measurements were recorded using a high-pressure shock tube (HPST) for stoichiometric fuel in air mixtures at reflected shock pressures (p5) of 20–30 bar and at temperatures (T5) of 987–1420 K. The current results together with rapid compression machine (RCM) measurements in the literature show that higher concentrations of the higher n-alkanes (C4–C7) ∼1.327% in the NG7 blend compared to the NG6 blend result in the ignition times for NG7 being almost a factor of two faster than those for NG6 at compressed temperatures of (TC) ≤ 1000 K. This is due to the low temperature chain branching reactions that occur for higher alkane oxidation kinetics in this temperature range. On the contrary, at TC > 1000 K, NG6 exhibits ∼20% faster ignition than NG7, primarily because about 12% of the methane in the NG7 blend is primarily replaced by ethane (∼10%) in NG6, which is significantly more reactive than methane at these higher temperatures. The performance of NUIGMech1.2 in simulating these data is assessed, and it can reproduce the experiments within 20% for all the conditions considered in the study. We also investigate the effect of hydrogen addition to the auto-ignition of these NG blends using NUIGMech1.2, which has been validated against the existing literature for natural gas/hydrogen blends. The results demonstrate that hydrogen addition has both an inhibiting and a promoting effect in the low- and high-temperature regimes, respectively. Sensitivity analyses of the hydrogen/NG mixtures are performed to understand the underlying kinetics controlling these opposite ignition effects. At low temperatures, H-atom abstraction byO˙H radicals from C3 and larger fuels are the key chain-branching reactions consuming the fuel and providing the necessary fuel radicals, which undergo low temperature chemistry (LTC) leading to ignition. However, with the addition of hydrogen to the fuel mixture, the competition by H2 for O˙H radicals via the reaction H2 + O˙H ↔ H˙ + H2O reduces the progress of the LTC of the higher hydrocarbon fuels thereby inhibiting ignition. At higher temperatures, since H˙ + O2 ↔ Ö + O˙H is the most sensitive reaction promoting reactivity, the higher concentrations of H2 in the fuel mixture lead to higher H˙ atom concentrations leading to faster ignition due to an enhanced rate of the H˙ + O2 ↔ Ö + O˙H reaction.

An experimental and detailed kinetic modeling study of the auto-ignition of NH3/diesel mixtures: Part 1- NH3 substitution ratio from 20% to 90%

Ammonia (NH3) is considered as a good carbon-free hydrogen-carrier fuel and has gained extensive attention as a promising alternative fuel for internal combustion engine in recent years. To explore the application potential of the NH3 burned with diesel fuel in internal combustion engine, the auto-ignition delay times of NH3/diesel fuel blends were measured in a rapid compression machine at a wide NH3 energy fraction of 20%, 40%, 60%, 70%, 80%, and 90%, temperature range of 675–995 K, pressures of 20–50 bar, and equivalence ratios of 0.5–1.5. According to the variation of ignition delay times with temperature, three different combustion regimes for NH3/diesel mixtures can be determined. It is found that the typical NTC behavior and two-stage ignition process were only observed at the regime where diesel chemistry dominates. Both the first-stage and the total ignition delay times increase with rising NH3 energy fraction, but decrease with the increase of equivalence ratio. Then, an updated NH3/diesel kinetic mechanism was proposed by adding new cross reactions between diesel and NH3. Results show that the current mechanism exhibits an obvious improvement for ignition delay time prediction compared to the original mechanism, though it still fails to reproduce the ignition delay times of NTC range for the mixture containing 20% NH3. Further sensitivity analysis and the OH radical rate of production analysis indicate that the overestimation of the reactivity for the NH3/diesel fuel blends at NTC range is closely related to reaction NO+HO2NO2+OH. The promoting effect of NO species on the conversion of inactive HO2 radical into active OH radical at low temperature greatly accelerates the autoignition. In addition, the current mechanism does not include the cross reaction between the large NTC-related species (QOOH, OOQOOH, OQOOH, ROO) and the N-containing species (NO, NO2, NH2), especially for the NO related reactions, which may have a great impact on the IDT simulation of the NH3/diesel mixtures. Consequently, this work presents new ignition delay time measurements and mechanism optimization for NH3/diesel mixtures. Continuous works should be emphasized on these unclear reactions to further understand the combustion behavior of NH3/diesel fuel blends.

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.

Experimental and kinetic modeling study on auto-ignition properties of ammonia/ethanol blends at intermediate temperatures and high pressures

The auto-ignition properties of ammonia (NH3)/ethanol (C2H5OH) blends close to engine operating conditions were investigated for the first time. Specifically, the ignition delay times (IDT) of ammonia/ethanol blends were measured in a rapid compression machine (RCM) at elevated pressures of 20 and 40 bar, five C2H5OH mole fractions from 0% to 100%, three equivalence ratios (ϕ) of 0.5, 1.0 and 2.0, and intermediate temperatures between 820 and 1120 K. The measurements reveal that ethanol can drastically promote the reactivity of ammonia, e.g., the auto-ignition temperature with merely 1% C2H5OH in fuel decreases accordingly around 110 K at 40 bar as compared to that of neat ammonia. Moreover, the promotion efficiency of ethanol is higher than hydrogen and methane with a factor of 5 and 10 under the same condition. Different dependences of IDT on the equivalence ratio were observed with different ethanol fractions in the blends, i.e., the IDTs of the 5%, 10% and 100% C2H5OH in fuel decrease with an increase of ϕ, but an opposite trend was observed in the mixture with 1% C2H5OH. A new chemical kinetic mechanism for NH3/C2H5OH mixtures was developed and it is highlighted that the addition of cross-reactions between the two fuels is necessary to obtain reasonable simulations. Basically, the newly developed mechanism can reproduce the measurements of IDT very well, whereas it overestimates the reactivity of the stoichiometric and fuel-rich mixture with 1% C2H5OH in fuel. The sensitivity, reaction pathway, as well as rate of production analysis indicated that the ethanol addition to ammonia fuel blends provides key interaction pathways and enriches the O/H radical pool which further promotes the auto-ignition process.

Shock-tube study of the ignition and product formation of fuel-rich CH4/ozone/air and natural gas/ozone/air mixtures at high pressure

The influence of ozone on the ignition delay times and product formation of methane and natural gas was determined in a high-pressure shock tube at fuel-rich conditions (ϕ = 2 and 10), ozone concentrations of 550 ppm, temperatures between 850 and 1700 K and pressures of about 30 bar. The addition of ozone causes a strong reduction of the ignition delay times for temperatures below 1100 K. Product concentrations were determined with fast sampling and GC/MS analysis for the experiments at ϕ = 10. Experiments and simulations show that CO, H2, H2O, C2H2, C2H4, C2H6, CO2, C3H6, and benzene are the main products. A comparison to similar studies with additives such as n-heptane, dimethyl ether, diethyl ether, dimethoxymethane and experiments without additives shows that C2H2, C2H4, and benzene concentrations are decreased, whereas CO concentrations are increased when ozone is used as additive. The experimental results were compared to simulations with chemical kinetics mechanisms from literature. The measured ignition delay times agree well with the simulations. A comparison of measured and simulated product distributions shows that the temperature dependence of the product formation is predicted well but that there are deviations of the absolute concentrations up to a factor of two for acetylene and benzene contrary to studies with other additives.

Heat Flux Measurement in Shock Heated Combustible Gases and Clarification of Ignition Delay Time

Correct understanding of the ignition and combustion processes in the combustion chambers are critical for modeling advanced schemes of engines of high-speed aircraft and promising spacecraft. Moreover, experimental data on the ignition delay time are a universal basis for the development and testing of combustion kinetic models. Moreover, the higher the temperature of the fuel mixture, the smaller this time value and the more important its correct determination. The use of a thermoelectric detector allows to measure ignition delay times and record heat fluxes with a high time resolution (to tenths of μs) during ignition in propane–air mixtures. Due to the faster response time, the use of it allows refining the ignition delay time of the combustible mixture, and the detector itself can serve as a useful device that allows a more detailed study of the ignition processes.

The effect of the addition of nitrogen oxides on the oxidation of propane: An experimental and modeling study

The oxidation of propane (C3H8), with the addition of different oxides of nitrogen (NO, NO2, and N2O) in concentrations of 0 – 2000 ppm, has been investigated for stoichiometric mixtures, at compressed temperatures of (TC) = 690 – 1420 K, and at compressed pressures of (pC) = 2.0 – 3.0 MPa using both a rapid compression machine and a high-pressure shock tube. These new ignition delay time (IDT) measurements, together with C3/NOx data available in the literature, provide a direct validation of NUIGMech1.3 which includes an updated C3/NOx sub-mechanism. The experimental results show that the mixtures with NO2 and NO/NO2 added have longer IDTs, inhibiting reactivity at TC〈 800 K, and shorter IDTs, promoting reactivity at TC〉 800 K, compared to the base C3H8/‘air’ mixtures indicating the complex chemical interactions involved. Both the inhibiting and prompting effects depend on the concentrations of NO and NO2 added and on the temperature regime. The addition of 1000 ppm NO2 significantly reduces the negative temperature coefficient (NTC) behavior of C3H8 in the temperature range 715–800 K compared to the addition of 200 ppm. Model predictions with 1000 ppm NO added, assuming no conversion of NO to NO2, are significantly slower than for both the 0 and 1000 ppm NO2 addition cases at TC < 800 K. Although NO and NO2 addition have different impacts on C3H8 oxidation at low-, intermediate, and high-temperatures, the addition of 1000 ppm N2O did not show any chemical effect at the conditions studied. NUIGMech1.3, with the updated C3/NOx sub-mechanism, reproduces the sensitisation effect of NOx on C3H8 with generally good agreement. Sensitivity and flux analyses have been performed to highlight the key reactions controlling ignition. The analyses show that competition between the reactions Ṙ+NO2↔RȮ+NO and Ṙ +NO2 (+M) ↔ RNO2 (+M) governs NOx sensitization on propane ignition. The inhibiting effect of NO and NO2 addition to propane stems from the nĊ3H7+ NO2↔ nC3H7Ȯ + NO and nC3H7Ȯ2 + NO ↔ nC3H7Ȯ + NO2 reactions, which compete for nC3H7Ȯ2 radicals, reducing the rate of isomerization of nC3H7Ȯ2 into Ċ3H6OOH1–3 (RȮ2 ⇌ Q˙OOH).

A comprehensive experimental and kinetic modeling study of di-isobutylene isomers: Part 1

Di-isobutylene has received significant attention as a promising fuel blendstock, as it can be synthesized via biological routes and is a short-listed molecule from the Co-Optima initiative. Di-isobutylene is also popularly used as an alkene representative in multi-component surrogate models for engine studies of gasoline fuels. However, there is limited experimental data available in the literature for neat di-isobutylene under engine-like conditions. Hence, most existing di-isobutylene models have not been extensively validated, particularly at lower temperatures (< 1000 K). Most gasoline surrogate models include the di-isobutylene sub-mechanism published by Metcalfe et al. [1] with little or no modification. The current study is undertaken to develop a detailed kinetic model for di-isobutylene and validate the model using a wide range of relevant experimental data. Part 1 of this study exclusively focuses on the low- to intermediate temperature kinetics of di-isobutylene. An upcoming part 2 discusses the high-temperature model development and validation of the relevant experimental targets. Ignition delay time measurements for the di-isobutylene isomers were performed at pressures ranging from 15 – 30 bar at equivalence ratios of 0.5, 1.0, and 2.0 diluted in air and in the temperature range 650 – 900 K using two independent rapid compression machine facilities. In addition, measurements of species identified during the oxidation of these isomers were performed in a jet-stirred reactor and in a rapid compression machine. A detailed kinetic model for the di-isobutylene isomers is developed to capture the wide range of new experimental targets. For the first time, a comprehensive low-temperature chemistry submodel is included. The differences in the important reaction pathways for the accurate prediction of the oxidation of the two DIB isomers are compared using reaction path analysis. The most sensitive reactions controlling the ignition delay times of the DIB isomers under the pressure and temperature conditions necessary for autoignition in engines are identified.

Combustion reaction kinetics of biodiesel/n-butanol blends: Experiments in an ultrahigh-pressure rapid compression machine

Burning oxygenated hydrocarbon biofuels in engines is a viable path for saving energy and reducing carbon emissions. N-butanol and biodiesel are two representative biofuels and have attracted widespread interest. In this study, the blends of n-butanol and biodiesel (a waste cooking oil) with different n-butanol ratios (40%, 60%, 80% by volume) were adopted to study their autoignition characteristics in a newly developed ultrahigh-pressure rapid compression machine. The ignition delay times of the blends were precisely measured under wide pressures of 10/20/40/60 bar, equivalence ratios of 0.3/0.5/1.0, and a temperature range of 700–970 K. Experimental results show that the ignition delay time decreases with the increase of pressure and equivalence ratio at the investigated conditions regardless the blending ratios. It is also found that the ignition delay time becomes slightly longer with the increasing n-butanol ratio in the blends at temperatures below 820 K. However, as the temperature further increases, the ignition delay times of different blends get closer and has a crossover tendency. The composition of the biodiesel was quantitively analyzed and a surrogate fuel was developed. An optimized mechanism was proposed based on a documented detailed mechanism with 461 species and 18,217 reactions. Simulation results show that the optimized mechanism better captures the dependence of the measured ignition delay times on temperature, pressure, and blending ratios over the entire temperature range compared to the original mechanism. In the end, species evolution and sensitivity analysis were performed sequentially with the optimized mechanism to give kinetics insight into the chemical interaction between biodiesel and n-butanol. The experimental data and modeling results reported here provide a basis for understanding the combustion reaction kinetics of biodiesel/n-butanol blending fuels.

Experimental and Kinetic Modeling Study on High-Temperature Autoignition of Cyclohexene.

Cyclohexene is an important intermediate during the oxidation of cycloalkanes, which comprise a significant portion of real fuels. Thus, experimental data sets and kinetic models of cyclohexene play an important role in the understanding of the combustion of cycloalkanes and real fuels. In this work, an experimental and kinetic modeling study of the high-temperature ignition of cyclohexene is performed. Ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST). The studied pressures are 5, 10, and 20 bar; the equivalence ratios are 0.5, 1.0, and 2.0; and the temperatures range from 980 to 1400 K for IDT in HPST. It is shown that the IDTs of cyclohexene exhibit Arrhenius behaviors as a function of temperature, and the IDTs decrease as the equivalence ratio and pressure increase. The experimental results are simulated using three previous detailed kinetic mechanisms and an updated detailed mechanism in this work. The updated detailed kinetic mechanism exhibits good agreement with experimental results. Reaction path analysis and sensitivity analysis are performed to provide insights into the chemical kinetics controlling the ignition of cyclohexene. The results demonstrate that different detailed kinetic mechanisms are significantly different, and there are still no unified conclusions about the major reaction path for cyclohexene oxidation. However, it is worth noting that the abstraction reaction by oxygen at the allylic site and the submechanism of cyclopentene are of significant importance for the accurate prediction of IDTs of cyclohexene. The present experimental data set and kinetic model should be valuable to improve our understanding of the combustion chemistry of cycloalkanes.

Experimental Investigation on Auto-Ignition Characteristics of Kerosene Spray Flames

To facilitate the better use of RP-3 kerosene in compression ignition engines, the auto-ignition behaviors of RP-3 kerosene spray were experimentally investigated in an optical rapid compression machine. Results show that most of the tests have good ignitability and combustion performance. For all the successful ignited cases, the flame kernel was found to be formed before the steep rise of pressure, which explained that image-based ignition delay time is always shorter than the pressure-based ignition delay time. The effects of ambient environment, injection pressure, and injection delay time on the pressure history, ignition intensity, combustion duration, heat release rate, and other parameters were investigated individually. The ambient environment has a strong influence on ignition delay time by accelerating the chemical reactions, whereas the high injection pressure helps the better vaporization of fuel spray. The effect of injection delay time is non-monotonic as the trade-off relation between heat loss and blending of fuel and oxygen. The heat release rate histories under different conditions were compared and analyzed, and the two-stage heat release phenomenon was observed in the negative temperature coefficient region. The ignition intensity region was determined based on the measured ignition delay times of RP-3 kerosene spray, and multiple linear regression correlation was used to study the ignition delay time sensitivity to multi-factors.

Comparative study on ignition characteristics of n-propylbenzene, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene behind reflected shock waves

Alkyl aromatics comprise a significant portion of real fuels. Among various alkyl aromatics, the C9H12 aromatic fuels including n-propylbenzene and trimethylbenzenes are representative alkyl aromatics, which are widely detected in real fuels, and they are widely employed as surrogate compounds in modeling real fuels. Thus, the combustion kinetic study of C9H12 fuels is necessary and urgent for fuel combustion. In this work, comparative experimental and kinetic modeling study of the high-temperature ignition of three C9H12 fuels is performed. New ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST) for 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene. The studied pressure is 2, 5 and 10 bar, the equivalence ratios are 0.5, 1.0 and 2.0, and the temperature range is from 1090 K to 1600 K for IDT in HPST. The experimental results are simulated using an updated detailed kinetic mechanism. Reaction path analysis and sensitivity analysis are performed to provide insight into the chemical kinetics controlling the ignition of the three C9H12 fuels. The present experimental data set and kinetic model results should be valuable to improve our understanding of the combustion chemistry of alkyl aromatics and to offer practical guidance for the surrogate model development of real fuels.

Combustion kinetics of [formula omitted]-propylamine: Theoretical calculations and ignition delay time measurements

To better understand the combustion chemistry and improve the model performance of aliphatic amines, the kinetics of H-abstraction reactions from n-propylamine (NPA) by H, CH3, OH, and NH2 radicals and the ignition characteristics of NPA are investigated. The CCSD(T)-F12/cc-pVTZ-F12 method is selected as the benchmark, and the performances of several density functional theory methods are evaluated by comparing the calculated reaction energies and barrier heights against the benchmark values. The M06-2X/jun-cc-pVTZ method is identified as the best one with a mean unsigned deviation of 0.59 kcalmol−1. With the selected method, the rate constants are calculated using the variational transition-state theory combined with the small-curvature tunneling and multi-structural torsional anharmonicity (MS-CVT/SCT) options at 298–2000 K. The H-abstraction reactions at the α-C site are the dominant channels for NPA + H/CH3/NH2, but this channel is less important for NPA + OH due to the significant variational effect. The ignition characteristics of NPA is studied in a shock tube at 1105–1441 K, 4–20 atm, and different equivalence ratios (0.5, 1.0, and 2.0). The ignition delay time of NPA decreases noticeably with increasing temperature and pressure, while it is insensitive to equivalence ratio. Based on our calculations, a literature model is updated and then assessed against the measured ignition delay times. The updated model shows a better agreement with the measurements but fails to predict the ignition behavior of NPA under certain conditions. The reaction pathway and sensitivity analyses demonstrate that the unsaturated aliphatic amines and imines (such as CH3CHCHNH2 and CH2NH) are the important nitrogen-containing intermediates, and further investigation of the subset of these species is needed to improve the combustion model of aliphatic amines.

Prediction of ignition delay times of Jet A-1/hydrogen fuel mixture using machine learning

To control the global warming trends, carbon footprint of all the human activities needs to be restricted, including the aviation industry. Mixing hydrogen with commercial kerosene jet fuels appears as a promising alternative fuel to reduce the carbon dioxide emissions of aviation engines. The addition of hydrogen can significantly impact the auto-ignition process of aviation fuels, which is a key ingredient of engine reliability. However, accurate calculations or measurements of ignition delay times (IDTs) over a wide range of pressures, temperatures and fuel blending ratios are complicated and time-consuming. To achieve real-time prediction of ignition delay time for hydrogen-blended jet fuels under various operating conditions, machine learning methods are introduced to build a data-driven proxy model in this work. First, the ignition delay times of Jet A-1/hydrogen fuel mixture are simulated using the well-known HyChem combustion reaction mechanism under different pressures, temperatures, equivalence ratios and blending molar ratios of hydrogen. After some validation against experimental results, an artificial neural network (ANN) model is trained using the database of ignition delay times. Furthermore, a sub-ANN is nested to the original ANN model as an improvement on certain local conditions. The results show that the single original ANN model leads to a large local relative error for IDT <103μs, generally referring to high temperatures and pressures conditions. With the help of the nested sub-ANN approach, the improved model achieves a significantly better accuracy for very fast ignition. Compared with other machine learning models such as random forest, the nested sub-ANN model is more efficient to predict the ignition delay times of Jet A-1/hydrogen fuel mixture, still mitigating computational cost. The sensitivity analysis shows how the nested sub-ANN model is less sensitive to the uncertainties of the input parameters than the original ANN model.

Laminar flame speeds and ignition delay times for isopropyl nitrate and propane blends

Laminar flame speed and ignition delay time measurements are presented for blends of isopropyl-nitrate and propane. The laminar flame speeds were measured in a spherical bomb, with the majority of the experiments performed at 373 K and 1 bar, with equivalence ratios between 0.6 <ϕ< 2.1; the fuel composition was 0%, 10%, and 100% isopropyl-nitrate. Additional experiments were performed at 300 K for pure propane as a fuel, and at 0.5 bar for pure isopropyl-nitrate as fuel. The ignition delay measurements were performed in a shock tube, with reflected shock temperatures between 1050 K and 1530 K and a nominal reflected shock pressure of 20 bar. Equivalence ratios of 0.5, 1.0, and 1.5 were considered for fuel blends with 0% and 1% isopropyl-nitrate in propane; additional experiments were performed with 10% isopropyl-nitrate at ϕ = 1. In comparison with the pure propane flames, the pure isopropyl-nitrate have a similar peak flame speed (SL0=60.3 cm/s at ϕ=1.09 for isopropyl-nitrate versus SL0=57.9 cm/s at ϕ=1.10 for propane), but the nitrate is considerably faster under fuel lean conditions, and it exhibits a much broader domain. The ignition delay times for 1% isopropyl-nitrate in propane are indistinguishable from the pure propane experiments for all three equivalence ratios. The 10% isopropyl-nitrate in propane mixture, in contrast, is significantly more reactive. Transition state theory calculations were performed for select RH + NO2 reactions. These calculations were added to a recently developed mechanism for isopropyl-nitrate and propane. Detailed simulations of the experiments were performed in Cantera. The agreement between the models and experiments is excellent. The largest discrepancies occur for the ignition delay times for 10% isopropyl-nitrate at T5<1150 K and for the flame speeds for ϕ>1.8, which suggests that further work needs to be done under these conditions. To the best of our knowledge, these data represent the first published results on the laminar flame speeds of any alkyl nitrate.

Experimental investigation to assess the efficacy of gasoline surrogates with engine testing

The formulation of effective gasoline surrogates is a challenging task due to advanced combustion strategies, engine design and variable operating conditions in spark-ignition engines. In earlier studies, the gasoline surrogates with iso-octane, n-heptane and toluene blends were designed to closely match the commercial gasoline fuels based on their laminar burning velocity variation and ignition delay time measurements. A new approach of proposing the next generation gasoline surrogates is investigated in the present study with direct testing of these surrogates in a real SI engine. The present study helps in assessing the efficacy of proposed gasoline surrogates in a real engine. The combustion, performance, and emission characteristics of proposed gasoline surrogates are compared with the commercially available gasoline fuels. A total of 10 (named A to F) surrogates were investigated with variable percentage of iso-octane, n-heptane and toluene. All the tested surrogates were successfully tested in a multi-cylinder engine. The result shows that, SF and SG are suitable to reproduce the combustion and emission characteristics of commercially available gasolines during detailed engine testing. It is observed that surrogate F and G results are more aligned with the commercial gasoline fuels thus representing the commercial gasoline more closely and can use further for validation and modeling of IC engines through the application of detailed kinetic models. However, a significant change was observed for other tested surrogates.

Reactive Molecular Dynamics Calculation and Ignition Delay Test of the Mixture of an Additive and 2-Azido-N,N-dimethylethanamine with Dinitrogen Tetroxide.

In order to shorten the ignition delay of 2-azido-N,N-dimethylethanamine (DMAZ) and dinitrogen tetroxide (NTO), four amines [tert-butylamine, pyrrole, N,N,N′,N′-tetramethyl ethylenediamine (TMEDA), and diethylenetriamine (DABH)] with a mass fraction of 5% were added to DMAZ, and the potential energy change and the product change during the reaction of the mixture of an additive and DMAZ with NTO were analyzed by Reactive molecular dynamics (ReaxFF MD) calculation. Then, the ignition delay of the mixture of the additive and DMAZ as well as pure DMAZ with NTO was measured by a drop experiment with a photoelectric sensor and high-speed camera. The results show that the addition of pyrrole greatly reduced the time to reach the maximum system energy and greatly increased the rate of HNO2 formation. The dripping of the fuel was approximately a uniform linear motion, and the expression was y = 43.13 + 7.16x. The ignition delay time recorded by the camera was in good agreement with that of the optical signal. The measured ignition delay time for DMAZ with NTO was 261.5 ms. The mixture of pyrrole and DMAZ with NTO had the shortest ignition delay time of 100 ms, and the proportion of shortening the ignition delay time was the largest. The results of the droplet experiment were consistent with those of ReaxFF MD simulation, indicating that HNO2 plays an important role in the ignition delay, that is, the formation rate of HNO2 is positively correlated with the ignition delay.

An optimized kinetic model for H[formula omitted]/CO combustion in CO[formula omitted] diluent at elevated pressures

Oxy-fuel combustion is a promising strategy for carbon capture. Existing syngas combustion models overpredicted recently measured ignition delay time data in high-level CO2 diluent and elevated pressures (40, 80, 100 and 200 atm). A trial model was constructed by updating the Kéromnès mechanism (Combust. Flame, 2013, 160, 995–1011) with some recent reaction rate studies and the addition of O+OH+M = HO2+M. Among all 33 reactions, 23 influential reactions were selected for optimization based on sensitivity results. 787 syngas shock-tube ignition delay time data and 1900 laminar flame speed measurements were considered. The optimized model was computed from the trial model using an efficient two-stage optimization strategy. Temperature-dependent initial uncertainty was implemented and all Arrhenius parameters were optimization simultaneously. The optimized model significantly improves the prediction accuracy for the ignition delay time targets, especially those in CO2 diluent, and produces comparable prediction performance for the laminar flame speed targets to the other models. Most of the reaction rate adjustments were well within the corresponding uncertainty (≤30%). Five reactions with relatively large rate adjustment were discussed. Sensitivity and reaction path analysis were conducted to examine the kinetic difference between the optimized model and the Kéromnès model at relevant conditions. Chemical and physical effects of CO2 as a diluent were investigated.

The effect of the addition of nitrogen oxides on the oxidation of ethane: An experimental and modelling study

This study reports new ignition delay time (IDT) measurements of ethane (C2H6)/‘air’ mixtures with NOx (nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O)) addition in the range 0 – 1000 ppm at stoichiometric fuel to air (φ) ratios, at compressed temperatures (TC) of 851 – 1390 K and at compressed pressures (pC) of 20 – 30 bar. In addition, new IDT measurements of three highly diluted C2H6/NO2 mixtures at φ = 0.5, TC = 805 – 1038 K, and pC = 20 – 30 bar are also studied. These new experimental data, together with data already available in the literature, are used to validate NUIGMech1.2 with an updated NOx sub-mechanism. Although the addition of 200 ppm of NO or NO2 to ethane shows a minimal promoting effect, the addition of 1000 ppm significantly promotes its reactivity. The similarity of the effect of the addition of both NO and NO2 addition is due to the fast conversion of NO into NO2 in the presence of molecular oxygen. However, the 1000 ppm NO doped ethane mixtures exhibit ∼20% shorter IDTs compared to the NO2 blended ones. The addition of 1000 ppm of N2O exhibits no effect on ethane oxidation at the conditions studied. The NUIGMech1.2 predictions can reproduce the sensitisation effect of NOx on ethane with good agreement over a wide range of pressure, temperature, equivalence ratio, and percentage dilution. Sensitivity and flux analyses of C2H6/NOx are performed to highlight the key reactions controlling ignition over the different temperature regimes studied. The analyses show that there is a competition between the reactions Ṙ + NO2 ↔ RȮ + NO and Ṙ + NO2 (+M) ↔ RNO2 (+M). This governs NOx sensitization on C2H6 ignition.

Development of a reduced n-heptane/toluene/unsaturated furans-PAH reaction mechanism for dual-fuel engine applications

Furan, 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) have gathered much attention in the research community. However, the reaction chemistry of the three unsaturated furans for the internal combustion engine (ICE) simulations remains scarce. Therefore, a reduced five-component reaction mechanism contains furan, MF, DMF, n-heptane and toluene was proposed for dual-fuel engine applications in present research. First, a detailed reaction mechanism for furanic fuels was reduced under a wide range of enginelike conditions by using combined reduction methods. Second, the reduced mechanism of furanic fuels was combined with an existing n-heptane/toluene mechanism containing the formation of polycyclic aromatic hydrocarbon (PAH) up to C16 to construct a combined mechanism. Third, the combined mechanism were optimized by adjusting the reaction rate constants of some significant reactions and the newly proposed reduced n-heptane/toluene/unsaturated furans-PAH mechanism comprising 98 species taking part in 253 reactions. Subsequently, the proposed mechanism was compared and evaluated with experimental measurements of ignition delay times and species mole fractions acquired from published papers. Meanwhile, new dual-fuel engine experiments were performed in a single-cylinder diesel engine, the experimental results were also used to validate the proposed mechanism. Overall, the predicted results obtained by this proposed n-heptane/toluene/unsaturated furans-PAH mechanism are in reasonable agreement with the available experimental data.